In clinical practice, diagnosis is performed using X-ray computed tomography (CT) imaging. In an X-ray CT system, the imaging subject is irradiated with X-rays to obtain projection data based on the X-rays that pass through the imaging subject. This projection data may be referred to as a projection image.
In an X-ray CT system, at first, the imaging subject is irradiated with X-rays in small doses so that X-ray images (scanograms) are obtained as a result of transmission through the imaging subject (scanogram scan).
An engineer performs setting of imaging area, i.e., alignment of the imaging area and setting of the size of the imaging area based on the X-ray images obtained in the scanogram scan. By irradiating this imaging area with prescribed doses of X-rays, a projection image is obtained (main scan). This projection data is used in image reconstruction.
Moreover, in the following description, X-ray images are sometimes known as scanogram data.
The imaging subject includes sites accompanied with movement of the lungs or liver and other sites. Depending on these sites, it is necessary to set conditions for imaging/reconstruction.
For example, in an X-ray tomography to a heart, at first, X-ray images are captured in order to align the imaging area. Next, in order to distinguish the cardiac from other sites, it is necessary to discriminate the tracheal bifurcation, etc., on the X-ray image, and set the area for each site.
In order to automatically discriminate the cardiac position and other sites to set conditions for imaging and/or reconstruction, the tracheal bifurcation is used to discriminate the upper edge of the heart. For this purpose, this tracheal bifurcation is extracted.
In order to extract this tracheal bifurcation, threshold processing is applied on current X-ray CT images and past X-ray CT images that have been imaged using an X-ray CT system for the same patient being diagnosed, extracting the tracheal bifurcation (the former art).
Moreover, in order to emphasize and extract the outline of the lungs and heart from the X-ray images, after a smoothing process is performed on the image, the profile is analyzed vertically facing the diaphragm, to extract the thoracic vertebral area (the latter art).
The latter art proposes extracting one of the lung fields from a chest X-ray image, seeking the lower edge of the lung field within this lung field in a Y-axis direction (vertical) profile starting from a prescribed position, and searching for the thoracic vertebral area from this position of the lower edge of the lung field, by means of an X-axis direction (horizontal) profile.
However, the former art relates to X-ray CT. With X-ray CT, there is no overlapping of organs, etc., in the direction of the X-ray transmission, and the radiation dose to be irradiated to the X-ray detectors increases. Thus, the image contrast intensifies.
Hence, threshold processing of pixel values allows the tracheal bifurcation to be extracted. However, this is the CT scan, so there are problems in that the scan time is longer than a scanogram scan, and the radiation doses of the specimen (patient) increase.
Moreover, the latter art disclose the same process of extracting the lung area, but aims at extracting the thoracic vertebrae in the end. In other words, the latter art does not disclose a method of extracting the tracheal bifurcation.